1. Field of the Invention
The present invention is broadly concerned with downhole-curable epoxy-based compositions for use in oil or gas well applications as a total or partial substitute for conventional casing cements. More particularly, the invention is concerned with such compositions and methods of use thereof, wherein the compositions include a predominant epoxy component including an epoxy resin and a glycidyl ether modifier, and one or more additional components, whereby the complete compositions may be injected into a well and cured therein at temperatures of from about 75-350° F. to create a compressible and resilient sealing body within the well.
2. Description of the Prior Art
Conventional oil and gas wells have to be drilled in multiple sections in order to ensure that the hydrostatic pressure in the section being drilled lies between the pore and fracture pressures of the surrounding formations. After drilling each section a casing is lowered into place and the annulus between casing and rock is filled with cement. Once set, the cement provides support and protection for the casing and should also provide a seal to prevent fluid communication between the formations through which the well has been drilled. However, cement often does not produce a reliable seal because it is a rigid and brittle material and because standard placement techniques do not provide a mechanism whereby it can be set into a suitable state of stress.
Conventionally, each subsequent section of the well has to be drilled at a smaller diameter in order to allow for its casing to be lowered down through the previous one, so that for very deep wells the reservoir can only be reached with a narrow diameter casing. This is a serious problem, because the smallest diameter, bottom-most casing limits the amount of hydrocarbons which can be recovered per unit time from the well.
Recently, expandable casings have been introduced into the market. These are lowered into the well and then expanded in situ to a larger diameter using a conical swaging mandrel moved from bottom to top. Using this technology the reduction in diameter with each successive section is reduced, and they can potentially be used to construct a well of constant diameter along its whole depth.
The use of solid expandable tubing in the oil and gas industry is becoming increasingly common. Such tubing offers the potential to drill deep wells of significantly larger diameter at the reservoir than can be done with conventional casings. However, expandable tubing poses new challenges for cementing. After casing expansion, the annulus between the casing and the borehole can be very narrow, which makes it difficult to achieve satisfactory mud displacement if cement is pumped at this stage. Therefore the cement is normally placed prior to the casing expansion. Accordingly, the expansion must be carried out after placement but before the cement has set; otherwise the rigid set cement will either be crushed by the expansion (potentially fracturing the surrounding rock) or will prevent expansion altogether. The cement must therefore have an unusually long thickening time, in as much as extra time must be allowed to carry out the expansion.
It is common practice to cement only the lower portion of each section in this way; if the whole annulus is cemented then an even longer thickening time is required, and this leads to excessively long wait-on-cement times in order to operate in a safe window. However, cementing only the lower section of the casing carries risks also, since it leaves the bulk of the casing unsupported and unprotected against corrosion by formation fluids, and reduces the probability of achieving zonal isolation.
Patent publication U.S. 2008/0105429 describes a method of cementing expandable well tubing making use of a sealant composition which is injected downhole and allowed to solidify into a deformable matrix. The composition may comprise poly(propylene oxide) diglycidyl ether and amine crosslinkers.
During well production, casing breaks sometimes occur along the length of the casing string. Unless promptly remedied, such breaks can drastically lower well production. It is common practice to inject additional well cement in an effort to plug the break, but this often gives only partial results.
There is accordingly a need in the art for improved downhole sealant compositions which can be used as a partial or complete replacement for conventional casing cements and which provides a compressible and resilient seal adjacent part or all of the casing.